ReturnPath's stuff is a bit more advanced than that. They give you a large list of email addresses to send to (consisting of a number of webmail clients and ISP

ReturnPath's stuff is a bit more advanced than that. They give you a large list of email addresses to send to (consisting of a number of webmail clients and ISPs). They then pick these up and provide stats on which have and haven't been delivered. They also ran your content through a number of spam filters (including, but not limited to SpamAssassin). They also provide a whitelisting service (previously the "Bonded Sender" programme) which helps deliverability in a lot of cases, but the primary benefit is that it's pretty much the only way to guarantee delivery of large quantities of email to Hotmail. So, they are closely involved with deliverability, but you're right in saying that it's down to Mailchimp to keep their sending IPs reputation clean.